A variety of thin film pigments have been developed for a wide variety of applications. For example, magnetic pigments are used in security devices as well as in decorative cookware. Color shifting pigments are part of anti-counterfeiting inks, automobile paints, and cosmetic preparations.
Color shifting pigments, colorants, and foils exhibit the property of changing color upon variation of the angle of incident light, or as the viewing angle of the observer is changed. The color-shifting properties of pigments and foils can be controlled through variation of parameters such as thickness of the layers forming the flakes and foils and the index of refraction of each layer. The changes in perceived color that occur for different viewing angles or angles of incident light are a result of a combination of selective absorption of the materials comprising the layers and wavelength dependent interference effects. The interference effects, which arise from the superposition of light waves that have undergone multiple reflections, are responsible for the shifts in color perceived with different angles. The reflection maxima changes in intensity and in position on the spectra, as the viewing angle changes, due to changing interference effects arising from light path length differences in the various layers of a material that are selectively enhanced at particular wavelengths.
The color shifting effects may be achieved by using pigment flakes, formed of multiple thin film layers so as to produce interference optical effects. Conventional 5-layer color-shifting flakes have the following symmetric design: absorber/dielectric/reflector/dielectric/absorber. The multilayer thin film structure may be formed on a flexible web material with a release layer thereon. The desired thin film layers are deposited on the web by methods well known in the art of forming thin coating structures, such as physical vapor deposition (PVD), sputtering, or the like. The multilayer thin film structure is then removed from the web as thin film color shifting flakes, which can be added to a polymeric medium such as various pigment vehicles for use as an ink or paint. When dispersed in a liquid or paste medium, the flakes may form color-shifting paint or ink, that may be subsequently applied to the surface of an object.
Eventually it was found that the use of color shifting pigment flakes may be further advanced by including a magnetic layer into a pigment flake. Such pigment flakes may be used for incorporation of a magnetic signature into a document. Aligning magnetic flakes with applied magnetic field may planarize the flakes in the binder so as to increase reflectivity of the coating, or may produce dynamic optical effects such as rolling bar or flip flop effects disclosed in U.S. Pat. No. 7,047,883 in the name of Raksha et al. incorporated herein by reference.
With regard to magnetic pigments, U.S. Pat. No. 4,838,648 to Phillips et al., incorporated herein by reference, discloses a thin film magnetic color shifting structure wherein the magnetic material, e.g. a cobalt nickel alloy, can be used as a reflector layer. However, such flakes provide less bright colors than similar flakes with an aluminum reflector.
EP 686675B1 in the name of Schmid et al. coats aluminum platelets with a magnetic material so as to form the following symmetric structure: oxide/absorber/dielectric/magnet/aluminum/magnet/dielectric/absorber/oxide. However, the overlying magnetic material downgrades the reflective properties of the pigment because aluminum is the second brightest metal (after silver), and thus any magnetic material is less reflective.
U.S. Pat. No. 6,838,166 in the name of Phillips et al., incorporated herein by reference, discloses color shifting pigment flakes with a magnetic layer sandwiched between two reflector layers formed of aluminum. The high reflectivity of the flakes is maintained because the duller magnetic material is hidden inside the reflector. U.S. Pat. No. 6,875,522 in the name of Seto et al., incorporated herein by reference, also discloses 7-layer flakes where a magnetic layer is hidden between two reflector layers, preferably made of aluminum. However, exposure of the flakes to an alkaline solution may result in decreased optical performance because aluminum corrodes in alkaline solutions.
A variety of applications may require chemical durability of pigment flakes or color shifting foils. For products and devices utilized for protection of security documents, there may be a requirement that the devices will function even after exposure to common environmental chemicals. The United States Department of the Treasury, when soliciting new security features and devices, specifically describes the desired level of chemical durability (“Counterfeit Deterrence Banknote Feature”, Solicitation Number: SSR-05-0004). The US Treasury lists three chemical reagents to be utilized in the testing of candidate security features: aqueous sulfuric acid (H2SO4), aqueous sodium hydroxide (NaOH), and aqueous sodium hypochlorite (“bleach”). The security feature must survive exposure to these three chemicals reagents (as well as exposure to other chemicals such as organic solvents).
Accordingly, there is a need for improved magnetic pigment flakes and foils that overcome or avoid the deficiencies of prior flakes and foils.